The invention relates to a color television display tube comprising an electron gun system of the "in-line" type. The system is disposed in an evacuated envelope for generating three electron beams whose axes are co-planar and which converge on a display screen provided on a wall of the envelope. The beams are deflected in the operative display tube across the display screen in two mutually perpendicular directions by means of a deflection unit comprising deflection coils producing a first and a second deflection field, the direction of the first deflection field being parallel to the plane. The electron gun system comprises correction elements of a magnetically permeable material positioned around the two outer beams at the end of the system closest to the display screen.
A color television display tube of this type is known from U.S. Pat. No. 4,196,370. A frequent problem in colour television display tubes incorporating an electron gun system of the "in-line" type is what is commonly referred to as the line and field coma error. This error becomes manifest in that the dimensions of the rasters scanned by the three electron beams on the display screen are different. This is due to the eccentric location of the outer electron beams relative to the fields for horizontal and vertical deflection, respectively. The Patent Specification cited above refers to a large number of Patents giving partial solutions. These solutions consist of the use of field shapers. These are magnetic field conducting and/or protective annular and plate-shaped elements mounted on the end of the gun and locally strengthening or weakening the deflection field or the deflection fields along part of the electron beam paths.
In colour television display tubes, various types of deflection units may be used for the deflection of the electron beams. These deflection units in tubes having an "in-line" electron gun system are mostly self convergent. One of the frequently used deflection unit types is what is commonly referred to as the hybrid deflection unit. It comprises a saddle line deflection coil and a toroidal field deflection coil. Due to the winding technique used for manufacturing the field deflection coil it is not possible to make the coil completely self convergent. Usually such a winding distribution is chosen that a certain covergence error remains, which is referred to as coma. This coma error becomes manifest, for example, in a larger raster (horizontal and vertical) for the outer beams relative to the central beam. The horizontal and vertical deflection of the central beam is smaller than that of the outer beams. As has been described, inter alia, in the U.S. Pat. No. 4,196,370 cited above, this may be corrected by providing elements of a material having a high permeability (for example, mu-metal) around the outer beams. The peripheral field is slightly shielded by these elements at the area of the outer electron beams so that these beams are slightly less deflected and the coma error is reduced.
Two problems then present themselves. The first problem is that the shielding of the outer electron beams also results in these beams being deflected to a lesser extent at the area where the field astigmatism is corrected in the field deflection coil. Since the (barrel-shaped=negative 6-pole) vertical deflection field can only perform an astigmatism correction by the grace of pre-deflection, the astigmatism correction of the field deflection coil becomes less. This can be corrected by positioning the electron gun as a whole further away from the screen and hence away from the coil, but this results in a display tube with a greater build-in depth. Another solution may be to provide an extra barrel-shaped component in the deflection field of the field deflection coil, but this causes the need for coma correction to be increased again. A second problem which presents itself is that the correction of the field coma (Y-coma) is anisotropic. In other words, the correction in the corners is less than the correction at the end of the vertical axis. This is caused by the positive "lensing" action of the line deflection coil (approximately quadratic with the line deflection) for vertical beam displacements. (The field deflection coil has a corresponding lensing action, but is does not contribute to the relevant anisotropic effect). The elimination of such an anisotropic Y-coma error by adapting the winding distribution of the coils is a complicated matter and often introduces an anisotropic X-coma.